Raw milk contains in addition to water (about 87%), fat (about 4.5%), lactose (about 5%) and protein (about 3.3%) also minerals and trace elements, such as calcium, magnesium, phosphorus, manganese, potassium, sodium, iodine and zinc. Further, milk contains also vitamins, such as A, K, B12, B6 and B2. The amino acid composition of milk proteins has better biological value than any other protein (red or white meat). Milk proteins contain a lot of so-called essential amino acids like lysine, leucine and isoleucine. The proteins of milk belong to casein or whey proteins. Typically the ratio of casein protein to whey protein in cow's milk is about 80:20. The major whey proteins in milk are beta-lactoglobulin and alpha-lactalbumin having molecular sizes of 14-18 kDa. Casein make up about 80% of the proteins in cow milk and are divided into alpha-, beta- and kappa-caseins. The molecular sizes of caseins are in the range of 19-23 kDa. In milk, casein exists in groups of molecules that are called micelles. These particles/micelles consist of casein, calcium, inorganic phosphate and citrate ions. In one casein particle there are about 20 000 single casein protein molecules. Casein particle has a porous structure containing water about 3 g per 1 g of casein.
Amino acids of animal- and plant-based proteins may be cross-linked by enzymes, such as transglutaminase, laccase, tyrosinase, peroxidase, sulfhydryl oxidase and protein glutaminase in a known manner. The transglutaminase enzyme (EC 2.3.2.13) catalyzes the generation of covalent linkages between the glutamine and lysine amino acid residues present in the protein molecules. When linkages are formed, ammonia is released. The enzyme was first used in Japan in the manufacture of surimi (seafish paste) products (Kuraishi, et. al., Food Rev. Int. 17(2), 2001, pp. 221 to 246). Covalent bonds formed in the enzyme treatment withstand different process conditions, such as heating and mixing, well. From milk proteins, caseins and particularly the κ-caseins, are the best substrate for transglutaminase. β-casein is also rich in glutamine and lysine, which are linked together by transglutaminase.
It is known that milk contains substances that inhibit the activity of transglutaminase. These inhibiting substances are deactivated in a heat treatment of milk. On the other hand, it is known that the content of said substances in relation to the total content of proteins and fat is reduced in ultrafiltration of milk.
Meat analogs based on plant proteins, especially soy proteins are commercially available. In recent years methods for preparing meat analogs from milk and egg proteins in addition to plant proteins have been developed and such processes are described in the patent publications EP 1467628 B1, EP 1588626 B1, EP 1643850 B1 and EP 1772060 B1, for example.